If you have not used ADS, how can you say that another tool is more accurate? Makes no sense to me.
This is complete nonsense. Momentum uses MoM for planar 3D modelling, HFSS uses FEM volume meshing for arbitrary 3D modelling. Both methods are a complete solution to the 3D EM problem. The difference is that Momentum is specialized on planar geometries and can not do arbitrary 3D objects. However, this has absolutely nothing to with the accuracy of the results. Users who think that HFSS will give more accurate results have been fooled by marketing bullshit.
---------- Post added at 09:53 ---------- Previous post was at 09:40 ----------
100% agreed.
Differences might be caused by different mesh density (set by the user) or different choice of ports (set by the user). For a similar "quality" of the mesh and similar port configuration, results for a planar antenna geometry should be very similar.